"How do you tell whether a camel is wild or domestic?"

As you will probably be shooting on private land the land owner will tell you if you can shoot camels, horses, scrub bulls etc or not.

If you are shooting on unrestricted unallocated crown land, open slather on all introduced species.

Regarding scrub bulls remember many large cattle stations (ranches) only operate on a proportion of their total size. Any bulls outside of that area are not mustered. Again the owner or manager will tell you if it is OK or not. And bulls can be aggressive irrespective of whether they are truly born wild or not.